The present invention relates to new and improved equipment for monitoring the yarn travel on a textile machine, particularly in the shuttle of a loom where the weft or filling yarn is drawn from a bobbin.
Various devices of the type using electromagnetic or piezoelectric transducers for sensing and monitoring the weft or filling yarn in the shuttle of a weaving loom have been described in patent literature. However, no such device has been introduced in practice in weaving mills. The reason for this failure is that conditions must be placed upon such weft monitors which cannot be met with the hitherto proposed means during the rugged weaving operations. First of all, these conditions impose permanent operating reliability over long working periods, requiring the sensing means arranged in the shuttle to have a high resistance to dust and corrosion, and further to the extraordinary accelerations which occur on picking and braking the shuttle and which tend to deform or even destroy the sensing means.
Another equally indispensable demand is that the sensing and monitoring device generates yarn travel signals large enough to safely stop the loom in the event of a yarn break, and to avoid undesired stops in the presence of spurious signals of the various types occurring under normal working conditions. Since the demand for ruggedness of the sensing device is inconsistent with the latter condition, it is extremely difficult to realize a satisfactory solution of the problem in question.
Some known weft or filling yarn monitoring devices or weft stop motions comprise electrical circuits disposed in the shuttle and provided with a contact which is held open by the traveling tightened yarn and closed when the yarn tension decreases, thus completing the electrical circuit and tripping a stop signal. Such devices, by way of example, are described in German patent publications Nos. 1,253,647 and 1,710,442. Contacts of that type are responsive to the high accelerations occuring on picking the shuttle, even in the case that the contacts are sealed against effects from outside. Moreover, any type of sensing device mainly responsive to the tension of the yarn is unapt for monitoring those phases of the shuttle flight when the tension of the weft or filling yarn is small.
Thus, sensing devices which have no contacts and are responsive to the travel of the yarn, particularly piezoelectrical yarn sensing and monitoring devices, are preferred. By way of example, such a device is known from U.S. Pat. No. 3,467,149 and Swiss Pat. No. 441,172. This known sensing device has a low mechanical resonant frequency in the range of 1 to 2 KHz and comprises a base member, a piezoelectrical crystal connected to the base member by two mounting elements, and an L-shaped thread feeler element attached at one of its ends to the base member and having a free arm in parallel relationship to the piezoelectrical crystal. The thread feeler element is mechanically coupled to the piezoelectrical crystal by a vibration coupling member. Additionally, an electrical resonant circuit tuned to a high-frequency of a few MHz is provided in the sensing device, comprising an induction coil and a variable capacitance diode. The piezoelectrical crystal is connected in parallel to the doide such that the capacitance characteristic of the diode changes at a rate corresponding to the vibration frequency of the thread feeler element or crystal. As a result, the resonant frequency of said high-frequency resonant circuit shifts at the vibratory rate. By such a shift, a second resonant circuit disposed in the lathe or sley beam and driven by a generator producing a high-frequency signal in the MHz range is influenced such as to produce a signal modulated by the low-frequency of the sensing device. That low-frequency modulation indicates the travel of the yarn in the shuttle.
The aforementioned patents do not discuss the problems of the ruggedness of the piezoelectrical sensing device and the suppression of spurious signals.